The typical hard disk drive includes a head disk assembly (HDA) and a printed circuit board assembly (PCBA) attached to a disk drive base of the HDA. The HDA includes at least one disk (such as a magnetic disk, magneto-optical disk, or optical disk), a spindle for rotating the disk, and a head stack assembly (HSA). The PCBA includes electronics and firmware for controlling the rotation of the spindle and for controlling the position of the HSA, and for providing a data transfer channel between the disk drive and its host.
The HSA typically includes an actuator, at least one head gimbal assembly (HGA), and a flex cable assembly. During operation of the disk drive, the actuator must rotate to position the HGAs adjacent desired information tracks on the disk. The actuator includes a pivot-bearing cartridge to facilitate such rotational positioning. The pivot-bearing cartridge fits into a bore in the body of the actuator. One or more actuator arms extend from the actuator body. An actuator coil is supported by the actuator body, and is disposed opposite the actuator arms. The actuator coil is configured to interact with one or more fixed magnets in the HDA, to form a voice coil motor. The PCBA provides and controls an electrical current that passes through the actuator coil and results in a torque being applied to the actuator.
Each HGA includes a head for reading and writing data from and to the disk. In magnetic recording applications, the head typically includes a slider and a magnetic transducer that comprises a writer and a read element. In optical recording applications, the head may include a mirror and an objective lens for focusing laser light on to an adjacent disk surface. The slider is separated from the disk by a gas lubrication film that is typically referred to as an “air bearing.”
The spindle typically includes a rotor including one or more rotor magnets and a rotating hub on which disks are mounted and clamped, a clamp attached to the rotating hub, and a stator. If more than one disk is mounted on the hub, the disks are typically separated by spacer rings that are mounted on the hub between the disks. Various coils of the stator are selectively energized to form an electromagnetic field that pulls/pushes on the rotor magnet(s), thereby rotating the hub. Rotation of the spindle hub results in rotation of the clamp, spacer rings, and mounted disks.
Excessive imbalance of the disk mounting hub, disk clamp, disks, and spacer rings (if any) of the spindle can cause undesirable disk drive vibrations and associated customer complaints. In extreme cases, such vibrations might even degrade the ability of the actuator to position the heads adjacent desired information tracks on the disk for reading and writing data. Therefore, it is advantageous to balance the hub, clamp, disk(s), and spacer rings (if any) of the spindle while or after they are assembled together.
In the environment of modern disk drive manufacturing, thousands of disk drive spindles need to be balanced each day, and so tools (typically automated to some degree) have been developed to facilitate this. Such tools may be capable of adding, removing, or moving one or more masses on the hub to counteract a net radial imbalance of the rotor (i.e. a net imbalance that would tend to dynamically translate the axis of rotation).
For example, such a balancing tool may measure an initial imbalance, and then select and affix a balancing ring of appropriate size and mass to the top of the disk clamp. Such balancing would counteract only a net radial imbalance. However, correction of net radial imbalance does not correct so-called “couple imbalance” where the imbalance causes a rotating moment to be applied to the spindle out of the plane of the disk. For example, such couple imbalance may be represented by equal and opposite radial imbalances at different heights along the spindle axis of rotation, so that the net radial imbalance is zero but nevertheless vibration is caused and emitted by the spindle and disk drive. Couple imbalance can cause Z-direction vibration to be emitted by the disk drive, where the Z-direction is a direction parallel to the spindle axis of rotation. Limiting Z-direction emitted vibration can be an important disk drive customer requirement.
A more complex balancing tool may correct couple imbalance by measuring an initial couple imbalance, and then selecting and affixing discrete balancing masses (e.g. plugs) at different heights through openings in the disk clamp and into selected holes in a pattern of holes in the spindle hub, the holes being at different angular positions relative to the spindle hub.
However, more complex balancing tools and methods tend to complicate both the manufacturing process and the disk drive design, and can represent substantial cost in a high-volume manufacturing environment. Moreover, the installation of balancing plugs to correct couple imbalance (a.k.a. multi-plane imbalance) may not be practical in smaller form factor disk drives, such as 2.5 inch form factor disk drives and smaller, because in such disk drives the walls of the spindle hub may be too thin. Thus, there is a need in the art for simpler and less expensive methods for reducing couple imbalance in a disk drive, and/or reducing vibration emitted by a disk drive.